1. Field of the Invention
The present invention relates to a negative electrode material for a secondary battery with a non-aqueous electrolyte, such as a lithium ion secondary battery, to a method for manufacturing the same, and to a lithium ion secondary battery by using the same, and the material is composed of a silicon-silicon oxide-lithium composite useful for the negative electrode material for a secondary battery with a non-aqueous electrolyte.
2. Description of the Related Art
Currently lithium ion secondary batteries are widely used for mobile electronic devices, such as a mobile phone, a laptop computer, and the like, because of high energy density. In recent years, with increasing awareness of environmental issues, an attempt to use this lithium ion secondary battery as a power source for an electric automobile, which is an environmentally-friendly automobile, has become active.
However, the performance of a current lithium ion secondary battery is insufficient for application to the electric automobile in terms of capacity and cycle durability. There has been accordingly advanced development of a next generation model of the lithium ion secondary battery that has high capacity and is superior in cycle durability.
As one problem of the development of the next generation model of the lithium ion secondary battery, improvement in the performance of a negative electrode material is pointed out.
Currently carbon negative electrode materials are widely used. The development by using a material other than carbon has also advanced to sharply enhance the performance, and a representative thereof is silicon oxide.
The silicon oxide has several times as much theoretical capacity as carbon has, and there is thereby possibility that silicon oxide becomes an excellent negative electrode material.
There were, however, problems such as low first efficiency, low electronic conductivity, and low cycle durability at the beginning of the development, and various improvements have accordingly made so far.
Here, the “first efficiency” means a ratio of discharge capacity to charge capacity in the first charge/discharge. As a result of the low first efficiency, the energy density of the lithium ion secondary battery decreases. It is considered that the low first efficiency of silicon oxide is caused by generating a lot of lithium compounds that do not contribute to the charge/discharge at the first charge.
As a method to solve this, there has been known a method of generating the above-described lithium compounds by making silicon oxide and lithium metal or a lithium compound (lithium oxide, lithium hydroxide, lithium hydride, organolithium, and the like) react in advance, before the first charge.
For example, Patent Literature 1 discloses use of a silicon oxide that can occlude and release lithium ions as a negative electrode active material, and a negative electrode material that satisfies a relation of x>0 and 2>y>0 wherein a ratio of the number of atoms among silicon, lithium, and oxygen contained in the silicon oxide is represented by 1:x:y.
As a method for manufacturing the above-described silicon oxide that is represented by a compositional formula of LixSiOy and contains lithium, there is disclosed a method in which a suboxide of silicon SiOy that does not contain lithium is synthesized in advance, and lithium ions are occluded by an electrochemical reaction between the obtained suboxide of silicon SiOy and lithium or a substance containing lithium. In addition, there is disclosed a method in which a simple substance of each of lithium and silicon, or a compound thereof are blended at a predetermined molar ratio, and it is heated under a non-oxidizing atmosphere or an oxygen-regulated atmosphere to synthesize.
It describes that as a starting raw material, each of oxides and hydroxides, salts such as carbonates and nitrates, organolithiums, or the like are exemplified, and although it is normally possible to synthesize at a heating temperature of 400° C. or more, a temperature of 400 to 800° C. is preferable, since a disproportionation reaction to silicon and silicon dioxide may occur at a temperature of 800° C. or more.
Moreover, Patent Literatures 2 to 4 describe that a chemical method or an electrochemical method is used as a method for preliminarily inserting lithium before storing the negative electrode active material in a battery container.
It describes that as the chemical method, a method of making the negative electrode active material directly react with lithium metal, a lithium alloy (lithium-aluminum alloy and the like), or a lithium compound (n-butyllithium, lithium hydride, lithium aluminum hydride, or the like), and a lithium insertion reaction is preferably performed at a temperature of 25 to 80° C. in the chemical method. Moreover, it discloses, as the electrochemical method, a method of discharging, at open system, oxidation reduction system in which the above-described negative electrode active material is used for a positive electrode active material and a non-aqueous electrolyte containing lithium metal, a lithium alloy, or a lithium salt is used for a negative electrode active material, and a method of charging oxidation reduction system that is composed of a non-aqueous electrolyte that contains a transition metal oxide containing lithium, the negative electrode active material, and a lithium salt, as the positive electrode active material.
Moreover, Patent Literature 5 discloses powder of silicon oxide containing lithium represented by a general formula of SiLixOy, in which the ranges of x and y are 0<x<1.0 and 0<y<1.5, lithium is fused, and a part of the fused lithium is crystallized. It also discloses a method for manufacturing the powder of the silicon oxide containing lithium, in which a blend of raw material powder that generates SiO gas and metallic lithium or a lithium compound is made to react by heating under an inert gas atmosphere or under reduced pressure at a temperature of 800 to 1300° C.
It discloses that, at that time, silicon oxide (SiOz) powder (0<z<2) and silicon dioxide powder can be used as the raw material powder that generates SiO gas, and it is used after adding reduction powder (metallic silicon compounds, and powder containing carbon) as needed. It also discloses that the metallic lithium and the lithium compounds are not restricted in particular, and as the lithium compounds, for example, lithium oxide, lithium hydroxide, lithium carbonate, lithium nitrate, lithium silicate, hydrates thereof, or the like can be used, other than the metallic lithium.
On the other hand, when electronic conductivity is low, the capacity of the lithium ion secondary battery under high load decreases, and particularly cycle durability decreases.
For improvement to enhance this electronic conductivity, Patent Literature 6 discloses a negative electrode material having an electronic conductive material layer formed on a surface of silicon oxide particles. It describes that the silicon oxide among them is silicon oxide having an elementary composition of Si and O, and is preferably a suboxide of silicon represented by SiOx (0<x<2), and that it can be lithium silicate in which silicon oxide is doped with Li. It also describes that a carbon material is preferably used for a conductive material and it can be manufactured by using a CVD method, a liquid phase method, or a sintering method.
Moreover, as one improving method to enhance the cycle durability, that is, to suppress an occurrence of a decrease in the capacity even when charge/discharge are repeated, Patent Literature 7 discloses a conductive silicon composite in which a diffraction peak attributable to Si (111) is observed when x-ray diffraction, the size of silicon crystal obtained by the Scherrer method based on the half-width of a diffraction line thereof is 1 to 500 nm, and a surface of its particle is coated with carbon, the composite having a structure that crystallites of silicon are dispersed to a silicon compound, particularly, the conductive silicon composite in which the silicon compound is silicon dioxide and at least a part of the surface thereof is adhered to carbon.
There is an example of a method for manufacturing this composite in which silicon oxide is subjected to disproportionation with an organic gas and/or vapor at a temperature of 900 to 1400° C., and carbon is deposited by chemical vapor deposition treatment.
Moreover, for improvement in both of the first efficiency and cycle durability, Patent Literature 8 discloses a silicon-silicon oxide composite doped with lithium, the composite having a structure that silicon particles having a size of 0.5 to 50 nm are dispersed to silicon oxide in an atomic order and/or a crystallite state, particularly, a conductive silicon-silicon oxide-lithium composite in which the surface thereof is coated with carbon at a coating amount of 5 to 50 mass % with respect to an amount of the whole composite particles after surface treatment.
It describes a method for manufacturing this composite, the method in which silicon oxide is a lithium dopant and lithium metal and/or an organolithium compound is used to dope with lithium at a temperature of 1300° C. or less, and further a method in which a silicon-silicon oxide-lithium composite that is pulverized into a predetermined particle size is subjected to heat CVD with an organic hydrocarbon gas and/or vapor at a temperature of 900 to 1400° C. and a carbon coating is formed at a coating amount of 5 to 50 mass % with respect to an amount of the whole composite particles after surface treatment.